The 26-year-old, who was capped during the 2009 of North America, left the region two summers ago after limited opportunities.
He joined London Irish but has returned to Swansea to link up with his former team-mates once again.
"It's good to be back," said Spratt, who has penned a two-year deal.
"Having worked with Steve (Tandy) and Gruff (Rees) before it was not a tough decision to make when the chance came to return.
"There has been a lot of change coaching-wise and with players moving on, but that happens with any professional rugby team.
"For me, it is a case of trying to push hard for rugby in the early part of the season, although I know there is tough competition in the centre with Andrew Bishop and Ashley Beck having been selected for the Wales squad in Australia and you have Tom Isaacs as well."
Spratt started 11 matches for the Exiles last season and hopes to build on his injury-free form at the Ospreys as he seeks further international honours.
"Obviously, any player would say the same, the aim is to play for your country, but my focus at the moment is on the Ospreys," added Spratt.
"It is a case of getting myself in good shape in pre-season, getting some game-time under my belt in France and against Bath and then hopefully playing a part when the league season begins.
"There is a real buzz about the place at the moment. I watched the Pro12 final against Leinster and it was a fantastic performance to beat the European champions on their own patch.
"We had a good start to the Pro12 last season which stood us in good stead and that is the aim again."